Schmitt trigger circuit and power supply monitoring apparatus

ABSTRACT

A Schmitt trigger circuit according to an embodiment includes a voltage dividing circuit that divides an input voltage and outputs a divided voltage, and a basic Schmitt trigger circuit that includes a transistor as a current controlling element and controls current flowing through a light emitting diode (LED) included in an external photocoupler on the basis of the output voltage of the voltage dividing circuit proportional to the input voltage. The voltage dividing circuit has a positive temperature coefficient.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to and incorporates by referencethe entire contents of Japanese Patent Application No. 2014-150658 filedin Japan on Jul. 24, 2014.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Schmitt trigger circuit and a powersupply monitoring apparatus.

2. Description of the Related Art

Recent years have seen the spread of electric vehicles. In suchcircumstances, a configuration has been developed in which electricpower is output from an electric vehicle battery to the outside so as touse the power for emergency.

In order to implement such a configuration, a power supply controlapparatus for electric vehicles has been developed. Such a power supplycontrol apparatus includes a mechanical relay that electrically connectsan electric vehicle battery for driving a vehicle to an outlet foroutputting power.

Such a power supply control apparatus has a voltage output function anda current output function so as to report a voltage or a supply currentof the vehicle battery to an external electronic control unit (ECU)while the power is supplied through the mechanical relay. Conventionaltechnologies are described in Japanese Laid-open Patent Publication No2014-033533 for example.

A vehicle battery for driving a vehicle is set for a relatively highvoltage (e.g., 275 V to 650 V), and thus needs to be electricallyinsulated from an output circuit (a communication interface) thatoperates at a relatively low voltage (e.g., 12 V) and communicates withan external ECU.

For this reason, a relatively high-voltage circuit and a relativelylow-voltage circuit are electrically insulated from each other using aphotocoupler.

In addition, a voltage fluctuation range of a vehicle battery may belarge, fluctuating from 40 V to 650 V for example. Furthermore, in orderto ensure stable operation of a power supply control apparatus, a lightemitting diode (LED) included in a photocoupler is driven by using aSchmitt trigger circuit having hysteresis so that monitoring results donot vary with some voltage fluctuation in a certain voltage range of avehicle battery that can supply power for emergency.

Meanwhile, a current proportional to a detected voltage flows through anLED included in a photocoupler, and an external ECU simply determineswhether a voltage equal to or larger than a certain voltage is detected.In consideration of the life of the LED or other conditions, it isdesired to limit current when a voltage of the vehicle battery is equalto or larger than the certain voltage. Improvement in the accuracy ofvoltage measurement has also been desired.

SUMMARY OF THE INVENTION

In view of the foregoing, the present invention provides a Schmitttrigger circuit that can improve the accuracy of voltage measurementwhile extending the life of an LED included in a photocoupler, and apower supply monitoring apparatus that uses the Schmitt trigger circuit.

It is an object of the present invention to at least partially solve theproblems in the conventional technology.

The above object of the present invention is achieved by the followingconfigurations.

According to one aspect of the present invention, a Schmitt triggercircuit includes a voltage dividing circuit configured to divide aninput voltage and output a divided voltage; and a basic Schmitt triggercircuit configured to include a transistor as a current controllingelement and control current flowing through a light emitting diode (LED)included in an external photocoupler on the basis of an output voltageof the voltage dividing circuit proportional to the input voltage. Here,the voltage dividing circuit has a positive temperature coefficient.

According to another aspect of the present invention, the voltagedividing circuit includes a Zener diode connected in a reversedirection, and a voltage-dividing resistor connected to a low-potentialside of the Zener diode.

According to still another aspect of the present invention, the Zenerdiode has a positive temperature coefficient.

According to still another aspect of the present invention, the basicSchmitt trigger circuit further includes a first NPN transistorincluding a base terminal connected to an output terminal of the voltagedividing circuit, and a second NPN transistor including a base terminalconnected to a collector terminal of the first NPN transistor. Here, theLED has a cathode terminal connected to a junction point between thecollector terminal of the first NPN transistor and the base terminal ofthe second NPN transistor.

According to still another aspect of the present invention, a powersupply monitoring apparatus includes a Schmitt trigger circuit includinga first voltage dividing circuit configured to divide an input voltagefrom an external power supply and output a divided voltage, and a basicSchmitt trigger circuit configured to include a second voltage dividingcircuit having a positive temperature coefficient and dividing an outputvoltage of the first voltage dividing circuit, and a transistor as acurrent controlling element, and control current flowing through a lightemitting diode (LED) included in a photocoupler on the basis of anoutput voltage of the second voltage dividing circuit; and an outputcircuit configured to include a phototransistor included in thephotocoupler and output power source monitoring information to outside.

The above and other objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of presently preferredembodiments of the invention, when considered in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a general structure of a power supplymonitoring apparatus according to an embodiment;

FIG. 2 is a diagram for explaining an exemplary circuit structure of aSchmitt trigger circuit;

FIG. 3 is a diagram for explaining operation of the Schmitt triggercircuit; and

FIG. 4 is a diagram for explaining another exemplary circuit structureof the Schmitt trigger circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following explains a preferred embodiment with reference to thedrawings.

FIG. 1 is a block diagram of a general structure of a power supplymonitoring apparatus according to an embodiment.

When a main battery BT1 as a vehicle battery for driving a vehicle isconnected to a charging outlet OL as a power supply terminal through arelay RL1, a power supply monitoring apparatus 10 detects and outputs asupply power current to an external ECU (a vehicle ECU) 60 and, whileinsulating the main battery BT1 that is a high-voltage system from theECU 60 that is a low-voltage system, detects and outputs a supply powervoltage to the ECU 60.

In this case, the power supply monitoring apparatus 10 turns on and offthe relay RL1 under the control of the ECU 60.

The power supply monitoring apparatus 10 mainly includes a power supplycircuit 11, a current detecting circuit 12, and a voltage detectingcircuit 13. The power supply circuit 11 stabilizes (regulates) powersupplied from an auxiliary unit battery BT2 for driving an in-vehicleauxiliary unit through the relay RL2 and supplies the power to theentire power supply monitoring apparatus 10. The current detectingcircuit 12 is connected to a current sensor CT for detecting a supplypower current of the main battery BT1 and outputs a current detectionsignal corresponding to the detected current. The voltage detectingcircuit 13 detects a voltage of the main battery BT1 through the relayRL1 and, while insulating the high-voltage system from the low-voltagesystem, outputs a voltage detection signal.

The current detecting circuit 12 includes a current sensor interface(IF) circuit 21, a reference voltage circuit 22, and an amplified outputcircuit 23. The current sensor IF circuit 21 supplies power for currentdetection to the current sensor CT and processes a signal detected bythe current sensor CT so as to output the signal as a current detectionsignal. The reference voltage circuit 22 generates and outputs areference voltage signal having a certain reference voltage. Theamplified output circuit 23 compares the voltage of the currentdetection signal output by the current sensor IF circuit 21 and thereference voltage signal output by the reference voltage circuit 22 andoutputs an amplified current detection signal to the ECU 60.

The voltage detecting circuit 13 includes a high-voltage dividingcircuit 31, a Schmitt trigger circuit 32, and an output circuit 33. Thehigh-voltage dividing circuit 31 serves as a first voltage dividingcircuit that divides and outputs a voltage of the main battery BT1applied through the relay RL1. The Schmitt trigger circuit 32 outputs avoltage detection signal CD of “H” level when the voltage divided by thehigh-voltage dividing circuit 31 is larger than a first thresholdvoltage, and outputs a voltage detection signal CD of “L” level throughan LED 48 included in a photocoupler FC when the divided voltage issmaller than a second threshold voltage that is smaller than the firstthreshold voltage. The output circuit 33 receives an input of thevoltage detection signal through the LED 48 and a phototransistor 61included in the photocoupler FC and outputs the received signal as avoltage detection signal for a low-voltage system to the ECU 60.

In the above-described configuration, the voltage detecting circuit 13has a function that outputs a voltage detection signal of “H” level tothe ECU 60 when the relay RL1 normally turns on and the voltage iscapable of supplying power.

The ECU 60 can determine whether the relay RL1 is normally operating(turning on and off) on the basis of a voltage detection signal outputby the voltage detecting circuit 13, and performs various control on thebasis of the voltage detection signal and an amplified current detectionsignal output by the current detecting circuit 12.

The following explains the Schmitt trigger circuit 32 in detail.

FIG. 2 is a diagram for explaining an exemplary circuit structure of aSchmitt trigger circuit.

The Schmitt trigger circuit 32 includes a voltage dividing circuit 41, afirst NPN transistor 42, a second NPN transistor 43, and a currentlimiting resistor 44. The voltage dividing circuit 41 serves as a secondvoltage dividing circuit that further divides the voltage of the mainbattery BT1 divided by the high-voltage dividing circuit 31. The firstNPN transistor 42 includes a base terminal to which the voltage dividedby the voltage dividing circuit 41 is applied, and an emitter terminalconnected to the ground. The second NPN transistor 43 includes a baseterminal connected to the collector terminal of the first NPN transistor42, and an emitter terminal connected to the ground and an outputterminal. The current limiting resistor 44 is connected to ahigh-potential side output line L31H of the high-voltage dividingcircuit 31 at one end and connected to the voltage dividing circuit 41at the other end.

Furthermore, the Schmitt trigger circuit 32 includes resistors 45, 46,and 47 and the LED 48. The resistor 45 is connected to a junction pointbetween the voltage dividing circuit 41 and the current limitingresistor 44 at one end, and to the collector terminal of the second NPNtransistor 43 at the other end. The resistor 46 is connected to ajunction point between the voltage dividing circuit 41 and the currentlimiting resistor 44 at one end, and to the collector terminal of thefirst NPN transistor 42 at the other end. The resistor 47 is connectedto a junction point between the resistor 45 and the second NPNtransistor 43 at one end. The LED 48 includes an anode terminalconnected to the other end of the resistor 47, and a cathode terminalconnected to a junction point between the first NPN transistor 42 andthe second NPN transistor 43, the LED 48 being included in thephotocoupler FC.

In the above-described configuration, the first NPN transistor 42, thesecond NPN transistor 43, the resistor 45, and the resistor 46constitute a basic Schmitt trigger circuit.

Note that the resistor 47 and the LED 48 are not included in an originalSchmitt trigger circuit, and included in an external circuit thatoperates by an output from the Schmitt trigger circuit. However, for theconvenience of explaining this configuration, the resistor 47 and theLED 48 are described as a part of the Schmitt trigger circuit.

The voltage dividing circuit 41 includes a Zener diode 51 and a resistor52. The Zener diode 51 has a cathode terminal connected to a junctionpoint between the current limiting resistor 44 and the resistor 45, andan anode terminal connected to the base terminal of the first NPNtransistor 42. The resistor 52 is connected to the anode terminal of theZener diode 51 at one end, and to the ground at the other end.

In the above-described configuration, in view of temperature property,the Zener diode 51 has been selected to have a positive temperaturecoefficient. In other words, a Zener diode having a property thatdecreases in resistance as temperature rises has been selected. In thisconfiguration, the temperature property of an anode-cathode voltage is4.4 mV/° C., for example.

By contrast, in view of temperature property, the first NPN transistor42 has a negative temperature coefficient. In other words, the first NPNtransistor 42 has a property that increases in resistance as temperaturerises. For example, the temperature property of a base-emitter voltage(Vbe) is −2.2 mV/° C.

Consequently, the fluctuation in emitter-collector current due to thefluctuation in ambient temperature is partly canceled out, and thusreduced, by the temperature property of the Zener diode 51, therebyimproving the accuracy of measurement performed by the voltage detectingcircuit 13.

The following explains operation performed in the embodiment.

FIG. 3 is diagram for explaining operation of the Schmitt triggercircuit.

In the following explanation, hysteresis of the Schmitt trigger circuit32 has been presented by setting, for an input voltage Vin of thehigh-voltage dividing circuit 31, a first threshold voltage Vth1 atwhich current starts flowing through the LED 48 having no current and asecond threshold voltage Vth2 (<Vth1) at which current stops flowingthrough the LED 48 having current.

The hysteresis is developed because values of currents flowing throughthe Schmitt trigger circuit 32 are different between immediately beforethe first NPN transistor 42 turns on and immediately before the firstNPN transistor 42 turns off and thus an on-threshold voltage Vth_on forturning on the first NPN transistor 42 differs from an off-thresholdvoltage Vth_off for turning off the first NPN transistor 42.

The reason why the currents flowing through the Schmitt trigger circuit32 are different between immediately before the first NPN transistor 42turns on and immediately before the first NPN transistor 42 turns off isbecause paths of the currents are different despite the fact that theoutput voltages of the high-voltage dividing circuit 31 immediatelybefore the transistor 42 turns on and immediately before the transistor42 turns off are substantially the same.

That is, the current path (a current value I1) immediately before thefirst NPN transistor 42 turns on is: the resistor 44→the resistor 45→thesecond NPN transistor 43, and the current path (a current value I2)immediately before the first NPN transistor 42 turns off is: theresistor 44→the resistor 45→the resistor 47→the diode (LED) 48→thetransistor 42.

In this case, by defining the voltage across the voltage dividingcircuit 41, that is, the output voltage of the high-voltage dividingcircuit 31 immediately before the first NPN transistor 42 turns on andthe output voltage of the high-voltage dividing circuit 31 immediatelybefore the first NPN transistor 42 turns off, as VX, and the resistancevalue of the resistor 44 as R44, the on-threshold voltage Vth_on and theoff-threshold voltage Vth_off are given by the following expressions:

Vth_on=R44×I1+VX,

Vth_off=R44×I2+VX,

where the current value I1=(the output voltage VX−a saturatedcollector-emitter voltage of the second NPN transistor 43)/theresistance value of the resistor 45, the current value I2=(the outputvoltage VX−a voltage drop VF of the LED 48−a saturated collector-emittervoltage of the first NPN transistor 42)/(the resistance value of theresistor 45+the resistance value of the resistor 47), and the currentvalue I1>the current value I2.

The calculations result in the on-threshold voltage Vth_on and theoff-threshold voltage Vth_off that are different by the following valuegiven by the following expression:

Vth_on−Vth_off=R44×(I1−I2)>0 ∵I1>I2,

and therefore,

Vth_on=Vth_off+{R44×(I1−I2)}.

Based on the difference, the hysteresis is developed.

In this configuration, the first threshold voltage Vth1 and the secondthreshold voltage Vth2 are defined as voltages each corresponding to abreakdown voltage (e.g., 8.2 V) of the Zener diode 51 depending on theoutput voltage of the high-voltage dividing circuit 31. In other words,when the output voltage of the high-voltage dividing circuit 31 is equalto or larger than the first threshold voltage Vth1 during voltage rise,a voltage corresponding to the breakdown voltage is applied between theanode and the cathode of the Zener diode 51, and when the output voltageof the high-voltage dividing circuit 31 is equal to or larger than thesecond threshold voltage Vth2 during voltage drop, a voltagecorresponding to the breakdown voltage is applied between the anode andthe cathode of the Zener diode 51.

The maximum value assumed for the input voltage Vin of the high-voltagedividing circuit 31 is a maximum voltage Vmax as illustrated in (a) inFIG. 3. A current that flows when the voltage is equal to or larger thanthe first threshold voltage Vth1 during voltage rise and a current thatis flowing when the voltage is equal to or larger than the secondthreshold voltage Vth2 during voltage drop are within the range of amaximum current ILEDmax that has been set in consideration of the lifeof the LED 48 and to be sufficient for detection operation of thevoltage detecting circuit.

The input voltage Vin of the high-voltage dividing circuit 31 should bea voltage proportional to the voltage of the main battery BT1. However,for facilitating the understanding of overall operation, the followingexplains a case in which the input voltage Vin (proportional to thevoltage of the main battery BT1) of the high-voltage dividing circuit 31rises to the maximum voltage Vmax at a constant rate of rise and thendrops to 0 V at a constant rate of drop. In an initial state, the inputvoltage Vin of the high-voltage dividing circuit 31 is 0 V, and the LED48 has no current.

(1) When the input voltage Vin of the high-voltage dividing circuit 31is smaller than the first threshold voltage Vth1 (a time t0 to a timet1)

First, a case is explained in which the input voltage Vin of thehigh-voltage dividing circuit 31 has not exceeded the first thresholdvoltage Vth1 and is smaller than the first threshold voltage Vth1.

In this case, the input voltage Vin of the high-voltage dividing circuit31 is applied, as a voltage (Vbe>transistor-on voltage=0.6 V), betweenthe base and the emitter of the second NPN transistor 43 through theresistor 44 and the resistor 46. Then, a certain current flows betweenthe collector and the emitter of the second NPN transistor 43.

Meanwhile, a voltage equal to or larger than a breakdown voltage (e.g.,8.2 V) is not applied to the Zener diode 51, and thus the first NPNtransistor 42 is off.

Consequently, when the input voltage Vin of the high-voltage dividingcircuit 31 is smaller than the first threshold voltage Vth1 asillustrated in (a) in FIG. 3, only a minute amount of current, which issupplied through the resistors 44, 45, and 47 and flows between the baseand the emitter of the transistor 43, flows through the LED 48 includedin the photocoupler as illustrated in (b) in FIG. 3. The LED 48therefore does not emit light.

As a result, an output signal of the output circuit 33 is at “L” level,as illustrated in (c) in FIG. 3, indicating that the input voltage Vinof the high-voltage dividing circuit 31 is in a state of being incapableof supplying power (in this case, the output voltage Vout is smallerthan the first threshold voltage Vth1).

In this case, the ECU 60 determines the state of the relay RL1 to be offbecause the input voltage Vin of the high-voltage dividing circuit 31 isin a state of being incapable of supplying power. If an output signal ofthe output circuit 33 is at “L” level after a certain time has elapsedsince an issuance of an instruction to turn on the relay RL1, the ECU 60determines that the relay RL1 is in an error state in which the relayRL1 cannot turn on. The ECU 60 then performs predetermined control forthe error, and causes a notification to be sent to a user on theabnormality of the relay RL1.

The ECU 60 determines that the relay RL1 is normal if an instruction toturn on the relay RL1 has not been issued or an instruction to turn offthe relay RL1 has been issued.

(2) When the input voltage Vin of the high-voltage dividing circuit 31is equal to or larger than the first threshold voltage Vth1 duringvoltage rise, and equal to or larger than the second threshold voltageVth2 during voltage drop (the time t1 to a time t2)

Next, a case is explained in which the input voltage Vin of thehigh-voltage dividing circuit 31 is equal to or larger than the firstthreshold voltage Vth1 during voltage rise, and equal to or larger thanthe second threshold voltage Vth2 during voltage drop.

In this case, a voltage equal to a breakdown voltage (e.g., 8.2 V) isapplied to the Zener diode 51, and a current proportional to the inputvoltage Vin flows through the Zener diode 51.

Then, the first NPN transistor 42 turns on, and as illustrated in (b) inFIG. 3, a substantially constant current=ILEDmax flows through the LED48.

The following explains the reason why the current ILEDmax issubstantially constant.

The current ILEDmax is given by the following expression:

ILEDmax=(Va−a voltage drop of the LED 48−a collector-emitter voltageV_(CE) when the first NPN transistor 42 is on)/(the resistance value ofthe resistor 45+the resistance value of the resistor 47)

where Va is a voltage at the junction point between the resistor 44 andthe resistor 45.

In an operational range of the Zener diode 51 when the input voltage isequal to or larger than a breakdown voltage, the voltage Va issubstantially constant.

Thus, the above-described expression gives the current ILEDmax beingsubstantially constant.

The term “substantially constant” is used because the current ILEDmaxgiven by the above expression slightly increases. This increase iscaused in the following manner: after breakdown of the Zener diode 51,as the output voltage of the high-voltage dividing circuit 31 furtherincreases, the voltage across the Zener diode 51 slightly increases(e.g., +0.1 V), which increases the base current of the transistor 42,and the base-emitter voltage of the transistor 42 slightly increases(e.g., +0.1 V), whereby the voltage Va slightly increases (e.g., 9.0 V).

However, as long as expected fluctuation in the input voltage of thehigh-voltage dividing circuit 31 is within the range between the firstthreshold voltage Vth1 (or the second threshold voltage Vth2) and thevoltage Vmax, the fluctuation in the base-emitter voltage of the firstNPN transistor 42 due to the fluctuation in the current flowing throughthe resistor 52 is within a range that does not cause large fluctuationin the current flowing through the LED 48 in a macroscopic viewpoint.

The voltage division ratio of the voltage dividing circuit 41 is definedto be 14.6:1, for example. That is, when the breakdown voltage of theZener diode 51 is 8.2 V, the voltage across the resistor 52 isapproximately 0.6 V. Such a voltage division ratio can decrease thevoltage division ratio at the high-voltage dividing circuit 31, andallows a smaller resistance value to be used for a voltage-dividingresistor included in the high-voltage dividing circuit 31, therebyreducing heat generation and enabling more accurate measurement.

Meanwhile, when the first NPN transistor 42 turns on, the base and theemitter of the second NPN transistor 43 are short-circuited. The secondNPN transistor 43 thus turns off and no current flows between thecollector and the emitter of the second NPN transistor 43.

The output signal of the output circuit 33 is at “H” level, asillustrated in (c) in FIG. 3, indicating that the input voltage Vin ofthe high-voltage dividing circuit 31 is in a state of being capable ofsupplying power (in this case, the output voltage Vout is equal to orlarger than the first threshold voltage Vth1).

Because the output signal of the output circuit 33 is at “H” level, thatis, the input voltage Vin of the high-voltage dividing circuit 31 is ina state of being capable of supplying power (in this case, equal to orlarger than the first threshold voltage Vth1), the ECU 60 determinesthat the relay is on.

If an output signal of the output circuit 33 is at “H” level after acertain time has elapsed since an issuance of an instruction to turn onthe relay RL1, the ECU 60 determines that the relay RL1 is in a normalstate in which the relay RL1 has normally turned on. In this case, theECU 60 performs control by determining whether current is normallyflowing on the basis of an amplified current detection signal output bythe amplified output circuit 23 of the current detecting circuit 12.

The ECU 60 determines that the relay RL1 is in an error state in whichthe relay RL1 cannot turn off (e.g., weld shut) if an instruction toturn on the relay RL1 has not been issued or an instruction to turn offthe relay RL1 has been issued. The ECU 60 then performs predeterminedcontrol for the error, and causes a notification to be sent to a user onthe abnormality of the relay RL1.

Furthermore, the ECU performs various control on the basis of a voltagedetection signal output by the voltage detecting circuit 13 and anamplified current detection signal output by the current detectingcircuit 12.

(3) When the input voltage Vin of the high-voltage dividing circuit 31has decreased smaller than the second threshold voltage Vth2 from avoltage equal to or larger than the second threshold voltage Vth2 (afterthe time t2)

When the input voltage Vin of the high-voltage dividing circuit 31 hasdecreased smaller than the second threshold voltage Vth2 from a voltageequal to or larger than the second threshold voltage Vth2, the inputvoltage Vin of the high-voltage dividing circuit 31 is applied, as avoltage (Vbe), between the base and the emitter of the second NPNtransistor 43 through the resistor 44 and the resistor 46. Then, acertain current flows between the collector and the emitter of thesecond NPN transistor 43.

Meanwhile, the voltage applied to the Zener diode 51 decreases smallerthan the breakdown voltage (e.g., 8.2 V) again and the first NPNtransistor 42 turns off again. Consequently, when the input voltage Vinof the high-voltage dividing circuit 31 is smaller than the secondthreshold voltage Vth2 as illustrated in (a) in FIG. 3, only a minuteamount of current, which flows between the base and the emitter of thetransistor 43, flows through the LED 48 included in the photocoupler asillustrated in (b) in FIG. 3. The LED 48 therefore does not emit light.

As a result, the output signal of the output circuit 33 is at “L” level,as illustrated in (c) in FIG. 3, indicating that the input voltage Vinof the high-voltage dividing circuit 31 is again in a state of beingincapable of supplying power (in this case, the output voltage Vout issmaller than the second threshold voltage Vth2).

Therefore, the ECU 60 determines the state of the relay RL1 to be offbecause the input voltage Vin of the high-voltage dividing circuit 31 isin a state of being incapable of supplying power.

The output voltage is varied in the above description. However, if theECU 60 controls the power supply monitoring apparatus to normally turnon the relay RL1 and the power of the main battery BT1 is sufficient, animmediate transition to the above-described state between the time t1and the time t2 should be made, whereby the ECU 60 can determine thatthe relay RL1 has normally turned on. By contrast, if the ECU 60controls the power supply monitoring apparatus to normally turns off therelay RL1 being on, an immediate transition to the above-described stateafter the time t2 should be made, whereby the ECU 60 can determine thatthe relay RL1 has normally turned off.

If the relay RL1 cannot transfer to an appropriate control state, theECU 60 detects that the relay RL1 is in an error state, performspredetermined control for the error, and causes a notification to besent to a user on the abnormality of the relay RL1.

As described above, according the present embodiment, the current thatflows through an LED included in a photocoupler for insulating ahigh-voltage system from a low-voltage system can be limited to acurrent equal to or smaller than the certain allowable maximum current(ILEDmax in the present embodiment), and therefore the life of the LEDincluded in the photocoupler can be extended.

Furthermore, a Zener diode connected in the reverse direction is used asa voltage-dividing resistor included in a voltage dividing circuit at aninput stage of the Schmitt trigger circuit 32. The Zener diode isselected to have a positive temperature coefficient with respect totemperature property, which is opposite to the temperature property(negative temperature coefficient) of a transistor including the baseterminal to which a voltage divided by the voltage dividing circuit isapplied. This configuration can improve the accuracy of voltagemeasurement.

Specifically, when the maximum voltage Vmax, which is the maximum valueof the input voltage Vin, is set to 300 V and an operating temperaturerange is set to −30° C. to 85° C., an actual voltage detection error is±2 to ±3 V with respect to the first threshold voltage Vth1 or thesecond threshold voltage Vth2 being a reference. This result indicatesthat the accuracy of the voltage detection has no problem with practicaluse.

The following explains a modification of the embodiment.

FIG. 4 is diagram for explaining another exemplary circuit structure ofthe Schmitt trigger circuit.

The Schmitt trigger illustrated in FIG. 4 is different from thatillustrated in FIG. 2 in that a voltage dividing circuit 41A, which is areplacement for the voltage dividing circuit 41, includes a third NPNtransistor 53 that is inserted at a junction point between the Zenerdiode 51 and the resistor 52.

In other words, the Schmitt trigger circuit illustrated in FIG. 4 isdifferent in that the third NPN transistor 53 is disposed, including abase terminal connected to the anode terminal of the Zener diode, anemitter terminal connected to one end of the resistor 52, and acollector terminal remained open.

The third NPN transistor 53 effectively functions as a diode having lessvoltage drop, allowing a minute amount of base current to flow.

As described above, in view of temperature property, when the Zenerdiode 51 is selected to have a positive temperature coefficient and thetemperature property of the anode-cathode voltage is 4.4 mV/° C., forexample, the first NPN transistor 42 and the third NPN transistor 53have negative temperature coefficients. For example, the temperatureproperty of the base-emitter voltage (Vbe) is −2.2 mV/° C.

Consequently, the fluctuation in the emitter-collector current of thefirst NPN transistor 42 due to the fluctuation in ambient temperature isfurther canceled out, and thus reduced, than in the case of the Schmitttrigger circuit illustrated in FIG. 2 by the temperature property of theZener diode 51 and the temperature property of the third NPN transistor53, thereby improving the accuracy of measurement performed by thevoltage detecting circuit 13.

The present embodiment exerts advantageous effects that can improve theaccuracy of voltage measurement while extending the life of an LEDincluded in a photocoupler.

Although the invention has been described with respect to specificembodiments for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art that fairly fall within the basic teaching herein setforth.

What is claimed is:
 1. A Schmitt trigger circuit comprising: a voltagedividing circuit configured to divide an input voltage and output adivided voltage; and a basic Schmitt trigger circuit configured toinclude a transistor as a current controlling element and controlcurrent flowing through a light emitting diode (LED) included in anexternal photocoupler on the basis of an output voltage of the voltagedividing circuit proportional to the input voltage, wherein the voltagedividing circuit has a positive temperature coefficient.
 2. The Schmitttrigger circuit according to claim 1, wherein the voltage dividingcircuit includes: a Zener diode connected in a reverse direction, and avoltage-dividing resistor connected to a low-potential side of the Zenerdiode.
 3. The Schmitt trigger circuit according to claim 1, wherein theZener diode has a positive temperature coefficient.
 4. The Schmitttrigger circuit according to claim 2, wherein the Zener diode has apositive temperature coefficient.
 5. The Schmitt trigger circuitaccording to claim 1, wherein the basic Schmitt trigger circuit furtherincludes: a first NPN transistor including a base terminal connected toan output terminal of the voltage dividing circuit, and a second NPNtransistor including a base terminal connected to a collector terminalof the first NPN transistor, and the LED has a cathode terminalconnected to a junction point between the collector terminal of thefirst NPN transistor and the base terminal of the second NPN transistor.6. The Schmitt trigger circuit according to claim 2, wherein the basicSchmitt trigger circuit further includes: a first NPN transistorincluding a base terminal connected to an output terminal of the voltagedividing circuit, and a second NPN transistor including a base terminalconnected to a collector terminal of the first NPN transistor, and theLED has a cathode terminal connected to a junction point between thecollector terminal of the first NPN transistor and the base terminal ofthe second NPN transistor.
 7. The Schmitt trigger circuit according toclaim 3, wherein the basic Schmitt trigger circuit further includes: afirst NPN transistor including a base terminal connected to an outputterminal of the voltage dividing circuit, and a second NPN transistorincluding a base terminal connected to a collector terminal of the firstNPN transistor, and the LED has a cathode terminal connected to ajunction point between the collector terminal of the first NPNtransistor and the base terminal of the second NPN transistor.
 8. Apower supply monitoring apparatus comprising: a Schmitt trigger circuitincluding: a first voltage dividing circuit configured to divide aninput voltage from an external power supply and output a dividedvoltage, and a basic Schmitt trigger circuit configured to include asecond voltage dividing circuit having a positive temperaturecoefficient and dividing an output voltage of the first voltage dividingcircuit, and a transistor as a current controlling element, and controlcurrent flowing through a light emitting diode (LED) included in aphotocoupler on the basis of an output voltage of the second voltagedividing circuit; and an output circuit configured to include aphototransistor included in the photocoupler and output power sourcemonitoring information to outside.